Sec. 5-2] 



RADIATION TRANSDUCERS 



297 



^_ 



N 



C' 



-Insulator 



-"- /■; 



Fig. (5-2)16. Geiger point counter. 



All-glass counters have been described by Maze. 1 Connection is 

 made by means of a layer of colloidal graphite ("Aquadag") 

 applied to the outside of the glass envelope. The effective resistance 

 of the glass wall is of the order of 10 8 ohms. 



The sensitivity to gamma rays can be increased by increasing 

 the surface area of the cathode, for instance, by employing screen or 

 metal gauze cathodes. The gain is moderate; an increase of the 

 surface area by a factor of 4 increases the relative sensitivity by 

 about 50 per cent. 



The lifetime of a self-quenching counter is limited by decomposi- 

 tion of the quenching agent. Good counters have a life of the order 

 of 10 10 counts; counters rilled with methane as a quenching agent 

 havealifetime of about 10 8 counts. At 

 the end of its useful life the operating 

 potential of the counter, the slope of 

 the plateau, and the number of spuri- 

 ous discharges increase, and multiple 

 pulses ("bursts") begin to appear. 

 Operation of the counter near the 

 lower limit of the plateau is recom- 

 mended to increase the lifetime. Because of the absence of decom- 

 position of the quenching agent, the lifetime of non-self-quenching 

 counters can be considerably longer. 



Design considerations and constructional details of counters for alpha-, beta-, 

 gamma-, and X-ray counters, as well as detectors for neutron recoils (n,a) and 

 (n,p) reactions and fission detectors, may be found in Rossi and Staub, op. cit., 

 chaps. 5-9. For further references, see D. H. Wilkinson, loc. cit.; S. A. Korff, 

 op. cit.; H. Friedman, Proc. IRE, 37, 791 (1949); D. R. Corson and R. R. 

 Wilson, Rev. Sci. Instr., 19, 207 (1948); G. J. Hine and G. L. Brownell, "Radia- 

 tion Dosimetry," Academic Press, Inc., New York, 1956; J. Sharpe, "Nuclear 

 Radiation Detectors," Methuen & Co., Ltd., London, and John Wiley & Sons, 

 Inc., New York, 1955. 



b. Point Counters. The counter type originally used by Geiger 

 is the point counter illustrated in Fig. (5-2)16. It consists of a needle 

 N held by an insulator within a cylindrical metal chamber C. An 

 open window in front of the needle point permits ionizing radiation 

 to enter the counter. The point counter is generally used in air at 

 atmospheric pressure; it can be used with the needle at a positive 

 or at a negative potential ; the discharge mechanism and the behavior 

 of the counter are, of course, different with different polarity. 



1 R. Maze, J. phys. radium, 7, 164 (1946). 



2 Evans and Mugele, loc. cit. 



